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Biology of Periodical Cicadas

No insects attract public attention like the periodical cicadas. They were the first insects to be noticed by settlers in the New World, and today, when they emerge, they receive national and international scrutiny.

What makes these insects so interesting is their unique life cycle and evolutionary strategy for survival. In a specific locality, every 17 or 13 years, they emerge in enormous numbers. The immature cicadas dig their way out of little holes, crawl up a vertical surface, split their nymphal skin, and slowly emerge as delicate white insects with red eyes.

emergence holes in ground
Emergence holes
wingless nymphs climbing tree trunk
Nymphs climbing up tree trunk
white adult emerging from skin
Molting nymphal skin

white, newly emerged cicada
Newly emerged cicada
hardened and darkened adult cicadas
Adult cicadas

tree twigs cut open by cicadas to lay eggs

tree twigs cut open by cicadas to lay eggs
browning leaves on trees due to cicada egg-laying
Scarred twigs and browning leaves as a result of cicada egg-laying

After about an hour, their wings begin to expand while their bodies slowly turn black. In some places, millions of cicadas will emerge in just a few days, satiating their predators as a species survival strategy.

About a week after their emergence, the male cicadas begin singing in order to attract mates. This orchestra dedicated to cicada sex can be so loud that it affects human behavior. Indeed, Indiana University nearly cancelled one of its graduation ceremonies because the cicadas were so loud that the speakers could not be heard.

After about a month's time, the cicadas are gone. In their wake, the trees look like they have been hit by a strong hailstorm, with the new twigs broken and hanging from the branches, their leaves brown and shriveled as the result of egg-laying by the females. The ground is covered with the remains of millions of cicadas, and soon our parks and yards start to reek with the odor of decay.

Meanwhile, a new generation of cicadas begins a secret life. After about 6 to 8 weeks, the eggs hatch within twigs and the minute cicadas fall to the ground and dig into the soil. There they will spend the next 17 or 13 years feeding from tree roots. They build a feeding chamber or tunnel 10 to 12 inches below the surface, where they stick their piercing mouthparts into the roots of trees to feed on the plant juices. As they age, they move closer to the surface, spending the last 7 or 8 years of their lives between 4 and 5 inches underground. One clue that they are there is the gradual increase in mole activity in areas where an emergence year is approaching.

two cicadas posterior end to posterior end
Cicadas mating
rows of cicada eggs in twig
Cicada eggs in twig
cicada larva
Cicada larva

Seven species of periodical cicadas are now recognized. The 17-year cicadas include Magicicada septendecim, M. cassini, and M. septendecula. The 13-year cicada species are called Magicicada tredecim, M. neotredecim, M. tredecassini, and M. tredecula. The suffix on the species names (-decim, -cassini, or -decula) is important because species that share the same suffix are more similar to each other in their color, size, and song than they are to the others. This similarity has led to questions about whether the 17-year and 13-year forms are truly distinct species or the same species on a different schedule.

The decim species are the largest, with bodies that are about 1.5 inches long, and they have extensive orange coloring on the underside of their abdomens. The cassini and decula species are both about one inch long, but the cassini are entirely black on their abdomens, whereas the decula species have narrow bands of orange. The species groups have different songs as well. The call of the decim species is often described as a hollow-sounding "pha-roah" that begins on a high note and drops in pitch on the second syllable. The song of cassini is a buzzing or clicking whir (sometimes compared to the sizzling of a hot skillet), and the song of the decula species a rhythmic clicking uncannily like a rotary sprinkler. Listen to some typical cicada vocalizations on Slide 7 of the Interactive Presentation.

Only male cicadas sing. The songs are produced by a pair of organs called tymbals, which are found on either side of the abdomen. Each tymbal consists of a ribbed membrane that stretches like a drumhead across an opening in the abdomen, which is hollow and acts as a resonating chamber to amplify the sound. Males gather in trees in chorusing centers, where they sing to attract female cicadas. Interested females fly into the trees and perch nearby, flicking their wings in response to the males' calls.

The males hear this flick and reply with another call, and females follow suit with another flick. As the pair draws closer together, the male changes his call slightly and mounts the female while tapping her with his foreleg. After mating, a female will lay her 400 eggs in the new growth of trees in clusters of ten to twelve eggs. After she has exhausted her supply of eggs, she will die. Within a week or two, all of the adult cicadas will be dead, but their offspring will continue the cycle, waiting another 13 or 17 years until they too emerge as adults.




Revised: December 16, 2008


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